This invention relates to a steam generating furnace adapted to burn high moisture content fuel and providing novel means and method for pre-drying the fuel as it enters the furnace.
Waste wood, as a by-product of the paper mill industry, has long been burned in furnaces to generate steam. Hogged or waste wood generally has a very high moisture content, being on the order of 50-70% water. In the past, in order to maintain stable burning conditions of waste wood, a secondary fuel such as oil or natural gas has been necessary. In recent years, as these fuels have become scarcer and more expensive, ways have been sought for burning wet waste wood with a minimum use of secondary fuels. Pre-drying the wood helps in this regard, causing it to burn faster and hotter with greater stability, higher efficiency and higher specific combustion rates.
One present means of drying high moisture fuel prior to burning it is to introduce it into the upper portion of a furnace, so that some of the moisture is removed as it falls downwardly through the furnace onto a grate at the bottom. Such an arrangement is shown in Glaeser, U.S. Pat. No. 2,483,728, and in Wood, U.S. Pat. No. 1,427,045. A major disadvantage of this arrangement is that the unburned fuel and pollutants are emitted from the furnace with the exhaust gases rather than being retained or returned to the furnace for complete combustion.
Fluidized bed furnaces, as described in Bryers, U.S. Pat. No. 3,893,426, are another means which have been used to burn high moisture content fuels. A characteristic of fluidized bed furnaces is that they must be operated at temperatures (1200.degree. F.-1800.degree. F.) significantly lower than those used in traditional combustion furnaces (2200.degree. F.-2400.degree. F.). The present invention is not applicable to fluidized bed furnaces.
It is, accordingly, an object of the invention to provide in a high moisture content fuel burning steam generating furnace of the traditional combustion type a novel means and method for predrying the wet fuel so as to increase the stability and efficiency of burning of the fuel while reducing the generation and release to the atmosphere of particulate and gaseous pollutants.